Question

An ideal gas has a volume of 3.0 L. If the number of moles of gas and the temperature are doubled, while the pressure remains constant, what is the new volume?

Answer 1

Pick a T (in Kelvin), number of moles, and pressure, calculate V using PV = nRT. Then double N and T and recalculate V.

Answer 2

I just pick any? Don't I have to put 3.0 L in the equation?

Answer 3

I thought it would be simpler and you could see how much it increased from one set of data to another.
To use 3L, Use PV = nRT
Set P =1, V = 3, solve for n, use R as 0.08206 and T = 200 (any T will do).
solve for n, the double n, and double T, and solve for V.

The easiest way to do it, logically, is
PV = nRT
use P = 1 and V = 3, then
V = nRT.
3 = nRT. Call n = 1, R is a constant, call T = 1, then 3 = R. Now double n to 2, T to 2 and substitute
V = 2*R*2 = 4V
V was 3 so it is now 12.
Now matter how you set up the numbers, you will always come back to 12 L for the answer.